Due to the doubts about the availability of and the supply with natural gas in the 1970s, considerable efforts were made to generate synthetic natural gas (substitute natural gas, SNG) proceeding from the large coal reserves known. This was discussed in particular everywhere where there was a large local demand of natural gas as important primary energy carrier and at the same time considerable coal reserves were available locally. The main constituent of SNG—like also in natural gas—is methane. As coal-based plants for generating SNG require a comparatively high investment, and subsequently large new natural gas reserves were discovered, which gave reason to hope for a long-term supply with inexpensive natural gas, the interest in the industrial generation of SNG initially declined again in the time following.
As the situation has changed to the effect that the end of the natural gas reserves known so far also is foreseeable, the interest in the methanation as an alternative source for natural-gas substitute gas has increased again in the recent past. In addition, the technology offers a possibility for utilizing large and remote coal reserves more efficiently. Geopolitical considerations also give rise to the desire to achieve greater independence of the comparatively few large natural gas reserves. The generation of SNG on an industrial scale therefore again meets with an increased interest. It is particularly advantageous that the infrastructure established for the supply with natural gas, for example already existing pipeline systems, can further be utilized practically unchanged.
As is explained in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 1998 Electronic Release, keyword “Gas Production”, the principle of the catalytic methane synthesis by hydrogenation of carbon monoxide (CO) with hydrogen (H2) dates back to papers by Sabatier and Senderens from the year 1902. The reaction can be described by the following reaction equation:CO+3H2═CH4+H2O
Carbon dioxide also can be converted to methane according to the equationCO2+4H2═CH4+2H2O
Both reactions are connected with each other via the CO conversion reaction (CO shift), which in the presence of active catalysts always proceeds simultaneously:CO+H2O═CO2+H2 
Both of said reactions for the formation of methane proceed strongly exothermally and with a decrease in volume. The formation of methane in a high yield according to the above reactions therefore is promoted at low temperatures and high pressures. To achieve acceptable reaction rates, the use of suitable catalysts then is required. Therefore, catalysts are used which are based on nickel as active metal component. The presence of catalyst poisons, such as for example sulfur-containing components, must carefully be avoided, since the deactivation of the catalysts used primarily depends on the presence of such catalyst poisons. Typical nickel-based methanation catalysts operate at temperatures of 300 to 700° C.; there are used for example catalysts with a high nickel content on special alumina carrier materials, which were stabilized by doping with zirconia.
Technical methods for producing SNG on an industrial scale proceeding from synthesis gas containing carbon monoxide and hydrogen have long since been known to the experts. For example, the U.S. patent specification U.S. Pat. No. 4,005,996 A teaches a method for increasing the energy content of a synthesis gas stream obtained by gasification of coal. The method includes the catalytic methanation of carbon oxides with hydrogen by means of highly active nickel catalysts, wherein a gas mixture containing methane and steam is generated in several reaction stages. The synthesis gas product of the gasification of coal initially is liberated from catalyst poisons and other impurities as well as a part of the contained carbon dioxide by gas scrubbing with suitable absorbents, for example methanol or absorbents containing amine. Depending on the composition of the primary gas from the gasification of coal, further conditioning stages, for example adsorption stages for removing sulfur-containing components on adsorbents containing zinc oxide, and additional conversion stages such as shift reactors are passed through for adjusting the hydrogen and CO content of the synthesis gas. The purified and conditioned synthesis gas then is heated up to the inlet temperature into the first methanation reactor of roughly 260° C. by heat exchange against recirculated product gas of the first methanation stage. The reactor pressure is about 25 bar(a). By admixing the recirculation gas to the fresh feed gas of the methanation, the gas composition also is changed advantageously such that in the catalyst bed and at the reactor outlet of the methanation no more deposition of solid carbon will occur. In addition, the recirculation of product gas serves for mastering the heat tonality due to the high exothermicity of the above-mentioned reactions. The first reaction stage of the methanation is followed by a further methanation stage, which is operated without product gas recirculation. The product gas of the methanation, which is enriched in terms of its methane content and thus its energy content, is cooled and dried and thus has a quality which is suitable for introduction or admixture into conventional natural gas pipelines. For introduction into a natural gas pipeline, the gas pressure of the SNG must be increased to the pipeline operating pressure by means of compression in a pipeline head station, which pressure can be up to 80 bar(a) according to Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 1998 Electronic Release, keyword “Natural Gas”, Chapter 4.1.1 “Pipeline Transmission”.
A more modern process variant for recovering SNG from synthesis gas is disclosed in the US patent application US 2009/0247653 A1. FIG. 2 of this document shows a process in which the synthesis gas initially passes through one or more methanation reactors, wherein a primary methanation product gas is generated, which subsequently is cooled, in order to separate water from the primary methanation product gas by condensation. A part of the methanation primary product dried in this way subsequently is recirculated as recirculation gas before the entrance to the methanation reactors. The remaining part of the primary methanation product gas is supplied as feed to a further adiabatic methanation reactor (“trim reactor”). Preferably, the process is carried out such that at least two series-connected primary methanation reactors are present, wherein the first reactor is charged with fresh synthesis-gas feed gas and the recirculation stream, and to the second reactor both the product gas of the first reactor and fresh synthesis-gas feed gas is supplied. In this process, too, a cooled and dried methanation product gas finally is obtained, whose pressure must be increased before its discharge into a pipeline network.
For transport to consumers, the SNG produced by means of methanation often is to be fed into an existing pipeline system. Due to the pressure loss suffered by the synthesis gas when passing through the methanation plant, and due to the lower pressure level in the methanation plant as compared to the pipeline pressure, it is required to compress the product gas rich in methane to pipeline pressure after the methanation plant. In the brochure “From solid fuels to substitute natural gas (SNG) using TREMP™”, available in the Internet under the web address www.topsoe.com, it is stated that it is frequently necessary to increase the pressure of the SNG produced before feeding the same into a pipeline system. Furthermore, it is stated that the pressure increase is effected after the production and drying of the SNG produced, i.e. directly before feeding the same into the pipeline.